1. Field of the Invention
The invention relates to a catalytic converter for the catalytic treatment of exhaust gas. The catalytic converter is provided in particular to purify and/or to detoxify exhaust gas from an internal combustion engine, for example a gasoline combustion engine, by a catalytic treatment, i.e. to free said exhaust gas from pollutants, by converting the latter by a chemical reaction. The internal combustion engine may belong, for example, to an automobile or other motor vehicle or may be used for stationary operation--for example for driving an emergency power generator.
2. Description of the Prior Art
Known catalytic converters have a housing with an inlet and an outlet for the exhaust gas. The housing contains gas-permeable catalyst means in which the exhaust gas is catalytically treated during operation of the catalytic converters. The catalyst means have, for example, a catalyst support frequently designated as substrate or a plurality of catalyst supports or substrates through which the exhaust gas flows in succession during operation.
European Patent Disclosure 0 514 326 discloses, inter alia, catalytic converters having housings which have a cylindrical lateral wall and two flat end walls and contain catalyst means having at least one annular catalyst support. The or each annular catalyst support has an outer surface, an inner surface and passages running from the inner surface to the outer surface. In the case of the catalytic converters shown in FIGS. 6 and 10 of the cited publication, the inlet communicates with an inner space enclosed by the inner surface of an annular catalyst support. During operation of these catalytic converters, the exhaust gas flows through the inlet into the inner space, is deflected outward therein and flows from the inside to the outside through the catalyst support into an outer, annular cavity. Investigations have shown that the flow rate and the flow density of the exhaust gas in the catalyst supports varies slightly along the axis, the distribution of the flow density also depending on the amount of exhaust gas fed in per unit time. The inhomogeneity of the flow distribution reduces the efficiency of the catalyst supports and also causes axially inhomogeneous heating, so that the catalyst supports have to have larger dimensions than would be the case with an exhaust gas flow homogeneously distributed over the axial extension of the catalyst supports. A further disadvantage of the catalytic converters drawn in FIGS. 6 and 10 of the cited publication is that their housings--based on the amount of exhaust gas to be treated--have a rather large diameter, which makes it more difficult to arrange the catalytic converters under an automobile. It would be possible for the outer and inner diameters of the annular catalyst supports to be smaller and in turn the axial dimension of the annular catalyst supports to be increased in size so that the volume of the catalyst supports remains constant. In this case, however, the axial inhomogeneity of the flow distribution in the catalyst supports is greater.
In the case of catalytic converter drawn in FIG. 5 of the cited publication, the inlet communicates with a deflection cavity which is connected to an annular, outer cavity present between the lateral wall of the housing and the outer surface of the annular catalyst support, which surface widens conically away from the inlet. During operation of this catalytic converter, the exhaust gas flows through the inlet into the deflection cavity, is deflected outward in this cavity, enters the outer cavity, flows into the catalyst support at the outer surface of said support, flows through the catalyst support into an inner space and leaves the catalytic converter through the outlet. The conical outer surface of the catalyst support improves the homogeneity of the flow density in the catalyst support. Since this consists of annular, corrugated sheet metal members having diameters changing gradually along the axis, the production of this catalyst support is, however, expensive. Furthermore, in the case of the catalytic converter according to FIG. 5 as incidentally also in the case of the catalytic converters according to FIGS. 1 to 3 and 7 to 9 of the cited publication, the exhaust gas can come into contact with large wall parts of the housing and, via these, release a relatively large amount of heat to the environment before it flows into the catalyst support. In the case of a so-called cold start, i.e. when the catalytic converter is approximately at ambient temperature at the beginning of the exhaust gas feed, this has the disadvantage that it takes a relatively long time for the catalyst support to reach the temperature required for efficient exhaust gas treatment.
The catalytic converter disclosed in German Patent Disclosure 2 201 881 has a housing with a cylindrical lateral wall which is connected, by transition sections tapering conically away from said lateral wall, to cylindrical connection pieces which serve as inlet and outlet for the exhaust gas. The housing contains a catalyst support having axial passages for the exhaust gas. The catalyst support is for the most part cylindrical but has, at its end located closer to the inlet, an end section which is in the form of a truncated cone and projects into the conical transition section of the housing. According to the drawing in this publication, the catalyst support has, at right angles to its axis and hence at right angles to the direction of flow of the exhaust gas flowing through it, a cross-sectional area which is only about four times as large as the cross-sectional area of the cavities bordered by the connection pieces. The exhaust gas therefore flows through the catalyst support at a relatively high velocity. The individual passages of the catalyst support must therefore have relatively large cross-sectional dimensions, since otherwise the flow resistance and the pressure loss are very large. However, the consequence of large cross-sectional dimensions of the individual passages is that the exhaust gas flowing through the catalyst support has only relatively little contact with those surfaces of the catalyst support which are coated with catalytically active material and border the passages. The passages must therefore be relatively long, which increases the flow resistance and pressure loss in an undesirable manner. Furthermore, the volume, the weight and production costs of the catalyst support are relatively high, based on the amounts of exhaust gas flowing through the catalytic converter per unit time. According to the last-cited publication, the truncated cone-like end section of the catalyst support is intended to distribute the exhaust gas flowing in through the inlet uniformly over the entire cross-sectional area of the catalyst support. However, the proprietor of the present patent application has performed numerical flow calculations for a similarly formed catalytic converter which show that the flow density in the catalyst support is still dependent on the distance from the axis and is lower in the outer cross-sectional regions than in the middle cross-sectional region. Since, according to the drawing of the last-cited publication, the conical transition section of the housing, which section is adjacent to the inlet connection piece, makes a rather large angle, namely an angle of about 30.degree., with the axis, there is furthermore the danger, in the case of high flow velocities, that the exhaust gas flow, on entering that region of the inner space of the housing which is bordered by the conical transition section, will become detached from its wall and will become turbulent, increasing the flow resistance.
The catalytic converter disclosed in British Patent Disclosure 2 128 893 has a hollow cylindrical catalyst support. The inlet of the catalytic converter communicates with the inner space enclosed by the cylindrical inner surface of the catalytic converter. The cylindrical outer surface and that end surface of the catalyst support which faces the outlet border an outer cavity connected to the outlet. The catalyst support is formed from a coil of an originally flat wire cloth and of a corrugated wire cloth and is gas-permeable both in the axial and in the radial direction. During operation of the catalytic converter, the exhaust gas flows into the catalyst support at the cylindrical inner surface of said support and then out of the catalyst support again partly at the cylindrical outer surface and partly at the flat end surface of the catalyst support. This catalytic converter has the disadvantage that the flow velocity and flow density of the exhaust gas flowing into the catalyst support at the inner surface of said support vary rather considerably along the axis. Furthermore, the exhaust gas covers distances in the catalyst support which are of different lengths and in some cases only very short and said exhaust gas is therefore catalytically treated only insufficiently in part.
The catalytic converter disclosed in U.S. Pat. No. 3,736,105 has a housing and catalyst means which have an inner and an outer, conical catalyst bed of annular cross-section. Each catalyst bed is bordered by perforated, conical lateral walls and has a particulate material arranged between said lateral walls. The inlet communicates with the inner space enclosed by the inner, conical lateral wall. Said inner space tapers away from the inlet but has, at its end opposite the inlet, a diameter which is still about 30% of the diameter of that end of the inner space which is connected to the inlet. The flow velocity and flow density of the exhaust gas flowing into the inner catalyst bed at the inner lateral wall of said catalyst bed varies rather considerably along the axis in the case of this catalytic converter too. The exhaust gas distribution is also dependent in particular on the velocity and amount of exhaust gas flowing through the inlet. Since the catalyst bed consists of a particulate material, the density and gas permeability of the bed may differ from place to place and may vary in the course of time.